Marketing and Supply Agreement with Eisai - Summary of Cost Sharing Allocation (Detail)	3 Months Ended
	Mar. 31, 2016 USD ($)
Belviq Product Pre Approval Up To One Hundred Million Dollars | Territories Outside of North and South America | Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate expense cap on equal split of development expenses	$ 100,000,000.0
Portion of expenses	50.00%
Belviq Product Pre Approval Up To One Hundred Million Dollars | Territories Outside of North and South America | Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%
Belviq Product Pre Approval More Than One Hundred Million Dollars | Territories Outside of North and South America | Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	100.00%
Belviq Product Post Approval Up To Fifty Million Dollars | Territories Outside of North and South America | Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate expense cap on equal split of development expenses	$ 50,000,000.0
Portion of expenses	50.00%
Belviq Product Post Approval Up To Fifty Million Dollars | Territories Outside of North and South America | Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%
Belviq Product Post Approval More Than Fifty Million Dollars | Territories Outside of North and South America | Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	10.00%
Belviq Product Post Approval More Than Fifty Million Dollars | Territories Outside of North and South America | Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	90.00%
Belviq Product Pre Approval | Non-US Territories in North and South America | Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	10.00%
Belviq Product Pre Approval | Non-US Territories in North and South America | Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	90.00%
Belviq Product Pre Approval Certain Stability Work | Non-US Territories in North and South America | Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%
Belviq Product Pre Approval Certain Stability Work | Non-US Territories in North and South America | Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%
Belviq Product Post Approval | Non-US Territories in North and South America | Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	10.00%
Belviq Product Post Approval | Non-US Territories in North and South America | Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	90.00%
Belviq Product Post Approval | UNITED STATES | Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	10.00%
Belviq Product Post Approval | UNITED STATES | Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	90.00%
Belviq Product Post Approval Certain Stability Work | Non-US Territories in North and South America | Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%
Belviq Product Post Approval Certain Stability Work | Non-US Territories in North and South America | Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%
Belviq Product Post Approval Certain Pediatric Studies | UNITED STATES | Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%
Belviq Product Post Approval Certain Pediatric Studies | UNITED STATES | Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%
Belviq Product Post Approval Non Fda Required Portion Of Cardiovascular Outcomes Trial Up To Eighty Million Dollars | UNITED STATES | Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate expense cap on equal split of development expenses	$ 80,000,000.0
Portion of expenses	50.00%
Belviq Product Post Approval Non Fda Required Portion Of Cardiovascular Outcomes Trial Up To Eighty Million Dollars | UNITED STATES | Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%
Belviq Product Post Approval Non Fda Required Portion Of Cardiovascular Outcomes Trial More Than Eighty Million Dollars | UNITED STATES | Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	100.00%
Lorcaserin Product Other Than Belviq Pre Approval | Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%
Lorcaserin Product Other Than Belviq Pre Approval | Arena GmbH | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate expense cap on equal split of development expenses	$ 250,000,000
Lorcaserin Product Other Than Belviq Pre Approval | Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%
Lorcaserin Product Other Than Belviq Post Approval Up To One Hundred Million Dollars | Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate expense cap on equal split of development expenses	$ 100,000,000
Portion of expenses	50.00%
Lorcaserin Product Other Than Belviq Post Approval Up To One Hundred Million Dollars | Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%
Lorcaserin Product Other Than Belviq Post Approval More Than One Hundred Million Dollars | Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	10.00%
Lorcaserin Product Other Than Belviq Post Approval More Than One Hundred Million Dollars | Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	90.00%